OK, got it. Generator Set, Engine Driven (Diesel), 7.0kW 28V DC (NSN 6115-415-3017) manufactured by Parker Keelavite of Warwick for Ptarmigan Packet Switch. It's got a 28V 300A alternator with possible regulator settings of 27.8V, 28.5V and 29.2V, held to within +/- 0.5V over normal speed and load ranges. It has indeed got a Hatz 4-stroke diesel engine.
Does that help?
Andy